Method and apparatus for measuring dielectric absorption in a high quality condenser



May-7, 1957 w. A. MENZEL ET AL 2,791,750

METHOD AND APPARATUS FOR MEASURING DIELECTRIC ABsoRPTIoN 1N A HIGH QUALITY coNDENsaR 3 .Sheets-Sheet 1 Filed OGt. 28, 1955 M. l .mm... mN

May 7, 1957 W. A. MENZEL ET AL ABSORPTION IN A HIGH QUALITY CONDENSER 3 Sheelzs-Sheen 5 Filed 001;. 28, 1955 FIGA.

VOLTAGE DIFFERENCES DIELECTRIC ABSORPTION OF AVITAMIN Q CONDENSER AT VARIOUS O-SOO AE IN voL'rs AFTER SosEc.

METHOD AND APPARATUS FOR MEASURING DIELECTRIC DIELECTRIC ABSORPTION vs TEMPERATURE VOLTAGE LOSS AFTER 30 SEC. CHARGED FROM O TO GOOVOLTS PROKAR INSULATION Loss 9 \v|TAM|N o \MYLAR MYLAR+PoLYsTYRENE PoLYsTYRENE l 94o o 4o so |20 |60 DEGREES F INVENTORS W. A. MENZEL.

ATTORNY United States -Pater-1t F METHOD AND APPARATUS FOR MEASURIYG DIELECTRIC ABSRPTION IN A HIGH QUAL- ITY CONDENSER Wolfgang A. Menzel, Silver Spring, Md., and Herbert E. Ruehlemann, Allentown, Pa., assignors to the United States of America as represented by the Secretary of the Navy Application October 28, 1955, Serial No. 543,632

11 Claims. (Cl. 324-60) (Granted under Title 35, U. S. Code (1952), sec. 266) 'The invention described herein may be manufactured. and used by or yfor the Government of the United States of America 'for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to means and a method of measuring dielectric absorption in high quality condensers and more particularly to the testing and measurement of such condensers during the development and production -thereof whereby the dielectric absorption of the condensers under various conditions of service may be determined with a high degree of precision.

With the modern development of precision electronic devices, the effect of dielectric absorption of condensers controlling the operation of such devices has assumed added signiiicance particularly with respect to the accuracy of operation thereof. The term dielectric absorption as employed herein may be dened as a certain apparent loss in the charge which is not available on the condenser plates after the charge or discharge current, as the case may be, has been interrupted. This dielectric absorption varies with the dielectric material employed in the construction of the condenser, the voltage applied 2,791,750 Patented May 7, 1957 this invention will be readily appreciated as the same Ibecomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a circuit in which the effects of dielectric absorption of a condenser are shown in dashed outline;

f Fig. 2 is a condenser testing circuit embodying the present invention in accordance with a preferred embodiment thereof;

Fig. 3, is a graphic illustration of the reduction in voltage of seven diierent types of condensers due to dielectric absorption over a thirty second period;

Fig. 4 is a diagrammatic showing of voltage loss of a vitamin Q condenser due to dielectric absorption as a function of applied voltage;

Fig. 5 is a graphic representation of voltage losses in dilerent condensers due Ito dielectric absorption during successive tests following a shorting operation; and Fig. 6 is a lgraphic representation of the effect of different temperatures with respect to ldielectric absorption of vtive different types of condensers.

Referring now -to the drawings for a more complete i understanding of the invention on which like numerals of thereto, the charge or discharge time and the tempera-4 p ture of the condenser.

The invention assumes added significance when ernployed with high quality condensers such, for example, as condensers having a leakage resistance of not less than l0u ohms lemployed in R-C timing circuits suitable for use with militaryweapons or for non-military u-se, as the case may be, in which the voltage at one or several condensers is required to `change with the passage of time in conformance with predicted nonlinear equations. Unknown dielectric absorption may appear as deviations from theoretical values and thus introduce errors from various factors and conditions Ywhich heretofore could not be predicted and controlled.

This invention provides a new and improved method and means 'for testing and measuring the dielectric absorption of condensers whereby knowledge of these factors and their effect may be beneficially employed in predicting circuit behavior and limitations. These results are achieved by the novel method of testing condensers and the apparatus suitable for making such tests disclosed herein as will be more clear-ly apparent as the description proceeds.

One of the objects of the present invention is the pro' vision of new and improved means for testing and measuring the dielectric absorption of a high quality condenser.

Another of the objects resides in the provision of a novel apparatus for testing and measuring the dielectric absorption of a condenser under different conditions of service.

A still further object is the provision of new and improved means for comparing the dielectric constant of a condenser to be tested with the dielectric constant of a condenser of which the dielectric constant is known.

Other objects and many of the attendant advantages of reference are employed to designate like parts throughout the several views and more particularly to Fig. 1 thereof on-which is shownv a condenser C charged by a voltage Eo, the characteristics of the condenser being shown as including a small resistance and inductance designated by the numerals R and L respectively. AThe condenser C also possesses the well known characteristic of dielectric absorption as illustrated on Fig. V1 by the dashed circuit including resistance RDA and CDA. The capacity element CDA has been diagrammatically illustrated to represent a loss of charge of the condenser vC due -to dielectric absorption by Ireason of a small portion of this charge owing into the theoretical condenser CDA-through a high resistance RDA. Duri-ng the condenser charging time a certain amount of charge is applied to condenser @DA by the power supply at Eo and the voltage loss on the plate of condenser C is dependent on the charging 'rtim-e.

I-t has been Ifound by actual test Ithat if the condenser C is charged from a low impedance source such for example, as a storage condenser of relatively large capacity for more than ten time constants, the theoretical voltage which could be expected at condenser C is defined as Eo which is 100 percent of the voltage applied at the voltage source. The actual voltage of the condenser measured after several minutes Ifollowing disconnection of the condenser from the charging source has been found to be somewhat -less than the charging voltage initially applied to the condenser and this voltage loss has been 'found to increase with decreasing charge time. The charge or discharge times of a condenser of this order of magnitude are commonly encountered in practical applications so that under conditions where a high degree of accuracy is required, for example in R-C timing circuits, dielectric absorption assumes considerable significance.

Thevoltage loss due to dielectric absorption, it should be noted, differs from voltage loss due to electrical leak.- age through the dielectric in that electrical leakage will cause the condenser to be eventually discharged after the charging circuit has been interrupted whereas dielectric absorption causes only a small percentage of reduction of the voltage of the condenser yas a result of the absorption of a small fractional portion `of the charge into the dielectric material.

A circuit suitable for use with the present invention is shown on Fig. 2 in which a pair of input terminals 11 are adapted for connection to a D. C. input voltage, for eX- V ample, of the order of 600 volts. A potentiometer 12 is connected acros the termin-als 11, a switch lprefera'bly' netted S1` and S2 respectively, are'provided, cachot the" switches vhaving seven contacts selectively engagea'ble by' the wiper arm thereof; contact 1 being connected tov ground, contacts 2t to 6' beingy connectedto taps' on the potentiometer 12, 'and contact 7A being connected to the contact of' switch' A n arrangement is thu's provided m whichthevoltage of thewip'er arms'ot switches Sl'and S2 may be adjusted in 1Q() volt stepsfrortitground"orpzero' voltagetovrh'e full D. C. inpntvouageselectively in-accordance'with the instant s'ettingo'f'l eachoftlie' switch wipers. l Y l Y v rthere isiy alsoprovideda'chargiiigswitchf S3 comprising three levels* or batiksj'eachbaulr having'iive contacts' selectively engageable by a Wiper.individualthereto: The three wipers are ganged' together aridfjnormallymngage contact 1' of their respectivek banks'1 inf whichL they are heldby-akey or latch '15* engaging anotch in the disc-l6g The disc is: quicklyjrotated in a counterclockwise directionas viewed in thel drawingbyaspiing' such as the spring 17 illustiated'as the key 15i's 'depressed thereby quick-ly'moving the wipers across the eontac'tsof their respective banks- 'uhtilthey engage their' contacts` Si Whenthis occurs, the wiper assembly' is'grough t'o" rest by a stop member lfengaging stoppini Theresetting ofthe switch to itsiniciar starr position' is' achieved'by knob 21 which; when manually actuated,v 'causes the wipers to be returned to'engagement with' their contacts 1` anidlatched in-this position'by the key latch 15.` When in this positionl further movement of knob' 21 and the wipers associated therewithl is prevented by asecondlstop member 22-`as the stop '18 movesinto engagement therewith.

Contact 1 of banks 1 and of switch SiI areu connected togeth'erand to resistance v23 by conductor 24 from which the circuitis/ continued by way of conductor `25 to theV wiper of switch S1 and to al voltmeter V, the other ter-` initialV of the voltmeterV being grounded to conductor v14.'

ba'nltsf and ofswitch S3 are unconnected and contactsffS'j of these banks are `Connected together by conductor 34'.'Y

Wiper 3 of switch S3l is connected to a standard polysty rene condenser C2 from' whence the circuit is continued by wayof resistance ZSto ground. Condenser CZ'has a capacity preferablypof l microfa'radand vemploysv a dielectric composed of polystyrene, the dielectric absorp-V tion of which is small an known. The capacity of condenser C1 is preferably the order ofS'O micro'farads. The resistance's-2f3 and 27 arer of the order of 5,000 ohms and resistance 28 ispreferably 100 ohms.

Contacts 1, v2, 3 and 4 of bankv 2 of switch S3 arc connected to ground at conductor 14 and contact 5 of this bankv is unconnected. Wiper l of switch S3 is con-k nected to test terminal 29 from whence the circuit is continued r`by way of the condenser to be tested, test terminal 31, resistancer3r2 and conductor 33 to wiper 2 of switch S3; A high sensitivity electrostatic voitm'eter such forexample, Vas, a Keithley vacuum tube electrometer model '200, is connected between conductorsfj and 14 andissrnornially shorted by wiper 2 of switch S3. Resistance 32 may be ofY the order ofV 100 ohms and the condenser'to be' testedA has a capacity equal preferably in the order of magnitude to the capacity of condenser C2, in the assumed case l microfarad although, itdes`ired'itr may be jofdifferent value. Oscillations in the Vcharge and discharge circuit of the condenser to be. tested are prevented byiresistance 32 in series therewith. Y

The operation of the circuit of Fig. 2 will now be deposition l microfarad condenser to be tested is connected to test terminals 29-31, that switch S2 has been moved into engagement with its contact 7 and switch S1 has been moved into engagement with its -contact- 2. LetV it furthermore be assumed that input terminals 111 are connectedito a source of 60() volts D. C., switch liis'closed and switch S3 is in the home position shown on the drawing. Under the conditionsv assumed, condenser. C1 ischarged through resistance 27 to a` potential ofc600 volts and condenser C2 and' the test condenser are each charged to a potential of 100 volts as evidenced by the readingA of voltmeters VllandV respectively,

The start key 15 isnow depressed. As the latch end of key 15 is disengaged' from' the' notch in disc 16, spring 17 causes the switch S3 wiperassernbly, disc 16e and knob 21 to bc moved quickly from the initial positionof the wipers in engagement' with their contacts 1" to a'na'l or rest position with the Wipers in' engagementwitli their contacts 5; Theswitch S3isso constructedand'arranged that the time of Contact ot its wipers withI contacts 2,' 3 and 4"'o'f their respective banks is'2` milliseconds as each contact ismomentarily engaged thereby. This time ot contact exceeds the normal charging time4 of condensers C2 and the test condenser by a factor' of` ten and thus the condenser C2'and the test condenser are'fully charged to 600 volts as theA switch arms move` across bank contacts 2. To`insure that these condcnsers will receivel their full charge 'regardless of a condition, of chatter or other faulty contact between wipers 1 and 3' with their respective contacts 2, contacts 2" of banks 1 and 3 are connected to their respective contacts 3, whereby there is no possibility of failure of the condenser C2Y and the test condenser to be fully charged when then switch wipers have passe'd rxiyondVr their respective contacts. 3'.

In the assumed example, storage condenser C1 isv charged t'o 600 volts, the capacity of condenserV C1 is 50 niicrofarads and' the capacity of condenser C2 andthe test condenser each is 1 microfarad. The potentiall of the charges on condenser C2 and the test condenser, therefore, are immediately brought to the charged value of nearly 600 volts on condenser C1 from an initial charge o'f 150()y volts', in the example assumed, as wipers 1 and 3 of' switch S3 engage their respective contacts 2. During movement of the switch wipers over their contacts 1 through v4, the electrostatic voltmeter, itv will' be noted, was' shorted by wiper 2 of switch S3 and as wiper 2 of switch'SfY moves out of engagement with its contact 4, this short circuit is removed. As the switch moves into its nal positioma circuit is closed from test terminal 31 by way of resistor 32 conductor 33 to the electrostatic voltmet'er from whence the circuit is continued by way of conductor 14, resistance 28, condenser C2, wiper 3l and the contact 5 of switch S3, conductor 34, Contact 5 and wiper 1 of switch S3 and thence to the other test' terminal 29 to'which the Vtest condenser is connected.

As the voltage of theV test condenser drops as the result of v'dielectric absorption, the voltage diierential between this Condenser and standard condenser C2 is made manifest bythe electrostatic voltmeter.

When it is desired to reset the tester circuit of Fig. 2, the charging switch S3 is restored to normal by Vknob 21 with the stop member 18 in engagement with stop pin 22 and, if desired, switch 13 maybe opened during or prior to this restoring operation.

Whereas in the foregoing example, the test condenser was charged from an initial voltage of volts to 600 volts, it will'be understood that this by way of example only asV the circuit of Fig. 2 is adaptable for various different tests such, for example, asapplying ground to both terminals of the condenser under test prior to the actuation of switch S3and that during' the travel of the switch S3 over its contacts 2 and'S the voltage of the test condenser may be suddenly `increased to a vpredetermined value' selectively in accordance with the instant setting '5 of switch S2. Furthermore, if desired, the condenser under test may be initially charged to any desired voltage prior to actuation of switch S3 and a selected voltage of lower value or ground, as the case may be, may be suddenly applied to the test condenser by switch S3 as the wipers thereof pass over contacts 2 and 3.

It has been found that condensers employing polystyrene as a dielectric possess dielectric absorption to a small or negligible degree and the charge or discharge time or previous history conditions of the condenser h as no effect on the time values and timing accuracy thereof. Although a polystyrene condenser will operate satisfactorily in timing circuits requiring a high degree of accuracy, it has been found that the application of such a condenser is restricted as the result of space limitations or possible exposure to higher temperatures. Several newly developed dielectric materials have recently been made available and their characteristics with respect to dielectric absorption have been tested and measured by the circuit of Fig. 2. The impedance of the electrostatic voltmeter employed with the circuit of Fig. 2 preferably has a value at least as high as 1014 ohms shunted by ap-v proximately 6 micromicrofarads. When therleakage resistance of the condenser under test is above 1012 ohms times microfarad, its influence on the instrument read* ings is negligible. When its value is lower, however, its influence must be separately determined. Condensers having ya leakage resistance of less than 1011 ohms times microfarad, however, should not be used in high quality R-C timing circuits and the insulation of the tester and of the reference condenser C2, therefore, is preferably in excess of 1013 ohms. The large storage condenser C1 is employed to charge both the standard or reference condenser C2 and the condenser under test for the reason that the impedance of a properly designed storage condenser is low and does not change with the life of the equipment. In the case of the discharge of both condensers from diiferent initial voltages to zero nal voltage, a storage condenser would not be required but for ease of operation and to obtain comparable results between charge and discharge tests the storage condenser is employed in both cases.

On Fig. 3 is shown various instrument readings obtained with the circuit of Fig. 2 versus time for seven different types of condensers. All of these condensers were charged from zero to 600 volts after an additional condenser shorting time of more than ten minutes. The curves shown on Fig. 3 represent the mean values of ten readings taken during a thirty second interval. In most of the condensers tested, the voltage loss, due to dielectric absorption, is close to its final value Within thirty seconds and it is our conclusion, therefore, that this time interval is suilicient to allow accurate determination of the quality of the condenser under test. It will be noted, however, that the two condensers having as a dielectric one sheet of paper added to polystyrene or Mylar respectively will reach an equilibrium state after an appreciably longer time than seconds and that the intial voltage increase is slower.

The polystyrene condenser, which was selected for these tests, possesses a small amount of dielectric absorption and the dielectric absorption of this condenser was found to be not the result of the polystyrene material but the result of minute air bubbles which were trapped in the condenser section. In each of the tests shown on Fig. 3 the various condensers were charged within two milliseconds from zero to 600 volts or vice versa and the figures appearing in the ordinate at the left of the figure as viewed in the drawing represent the voltage losses from the original value of 600 volts, the ligure 17.5, for example, representing a nal voltage of 582.5 at which the paper -i-polystyrene condenser was charged at the expiration of thirty seconds after the charging circuit had been interrupted.

It is now known that the voltage loss due to dielectric absorption does not reach its inal value in sei` onds. A longer time,therefore, which Varies appreciably with-various types of condenser dielectrics is necessary for absolute measurement. series of measurements is made in short intervals with the same condenser, a certainamount of -dielectric charge may remain in the condenser dielectric after each test. It therefore becomes necessary to short the condenser after each test for several minutes beforethe next test on the condenser is performed when extreme accuracy of measurement of the dielectric constant is to be obtained,

Referring now to Fig 4, on which is shown in graphic form changes in voltage loss due to dielectric absorption as a function of applied voltage, the six curves shown thereon illustrate the results obtained with a vitamin Q condenser. In the iirst series of tests, after an initial condenser shorting time of more than 10 minutes, the condenser was'charged from 0 to 100 volts and the charge was increased to 600 volts by increments of volts each. Within this voltage range the dielectric absorption was proportional to the incremental change in applied voltage. This condenser behavior was also conrmed in a second series of tests in which the condenser Was precharged to 300volts for more than 10 minutes and in which the voltage was then raised to 400, 500 and 600 volts, respectively. These tests clearly show that the change of voltage alone determines the amount of dielectric absorption and not the potential at the condenser. This is clearly illustrated in the case of 0 to 200 volts, for example, being equal to the change from 300 to 500 volts. Roughly linear relationship between voltage changes and dielectric absorption was observed in practically all of the condenserswhich were checked by the circuit of Fig. 2.

Since it is known that the voltage loss due to dielectric absorption does not reach its nal value within 30 seconds, a considerably longer time which varies with various types of condenserr dielectrics during which the condenser is shorted is necessary for absolute measure. On the other hand, when a series of measurements is made in succession during short intervals of time with the same condenser, a certain amount of dielectric charge may remain after each test on the condenser dielectric and it has been found necessary to short the condenser after each test for several minutes before the next test is performed to obtain a high degree of accuracy of measurements. On Fig. 5 is shown in graphic form the result of a series of tests with various shorting times on various types of condensers. From these curves it will be noted that the voltage loss due to dielectric absorption with the condensers shorted for various times ranging from ten minutes to nineteen hours before testing show only small deviations, the loss appearing greater when the condenser is shorted for a longer time. The same results were obtained with all the other types of condensers tested except in the case of a tested condenser using a combination of paper and polystyrene. This condenser had a high voltage loss on the first reading whenever the condenser was shorted for a long time before the test. After approximately five subsequent readings with ten minute shorting time therebetween, the voltage loss was appreciably smaller and fairly constant as illustrated on Fig. 5. From the foregoing it is clearly apparent that ten minutes shorting time is insuf' cient to discharge the dielectric when it has been previously exposed to the electric field for thirty seconds.

A series of tests were made on various types of condensers by employing the tester circuit of Fig. 2 with the condensers at room temperatures and using the method to discharge from a higher initial voltage to a lower final voltage. The condensers were shortedv before the tests for various shorting times ranging from ten minutes to nineteen hours. They were initially charged for 1.5 minutes at each testand readings were taken from the electrostatic voltmeter after the switch On the other hand, when a found .that thereis only asmall. deviation-due. tofvarying the,shortingtimefin this range.- rlhevoltage recoveryV however, is appreciably lower than the voltage loss in the-,test p'reyitnisly` described and the ratio oli-recovery voltage.. to .voltage loss; ,isdiierent. for eaclr different typeot. condenserv tested. This difference in recovery ratte;` obviously wasnotcaused bythe leakage resistance ofv the. dielectric for the reason-that allfof the condensers tested.withfa-single exception, had. an insulation resistance of more than'y 10.1l ohms. times microfarads at room-temperature; It isfouropinion thatthis diterence maybe satisfactorily explainedby noting that, in the case of, the charging measurements, the dielectric is subjected to-a highveld.duringtherentire measurement time of, thirty seconda, whilethe decrease `in lield strength because ofv voltage losstis' small. During this period, the dielectric iszbeingformedand is removing charge from the,v condenser plate. In the case.V of discharge measurements-only the small eld from the recovery voltage isfpresent duringthe time of the. measurement so that the-,forces tending to produce dielectric vrecovery are small: and' inl opposition -to thev chargeV accumulating on the condenser plate.

On Fig. 6isY shownc the dielectric absorption of live different condensers when exposed to diierent temperatures. Onthisgure is shown the voltage losses after thirty. seconds versustemperature when the condensers are'charged-,from zero to 600 volts. Readings at higher temperatures should be corrected in cases where the leakage resistance of the condenser had dropped below 1011 ohmstimesmicrofarad. Such a correction is shown by the dashed lines for the vitamin Q andProkar condensers. .All-;condensers with theV exception of the polystyrene condenser,` also show an increase in voltage loss at lower temperatures, which increase cannotbe satisfactorily eX- plained by increase leakage resistance which, for these high quality condensers should be negligible at the low temperature range. Precautions were taken to prevent small influencesresulting fromhurnidity at the terminals. It is believed; therefore, that a certain amount of this increase: in. dielectric absorption at lower temperatures is due to the fact that, atsuch lower temperatures, the dielectric absorptionprocess is slower so that a smaller percentage. of the dielectric absorption is covered during the charge cycle andia. larger percentage `occurs thereafter.

From. the foregoing` it will be clearly apparent that we have provided a new method of testing and measuring the dielectric absorption of a high quality condenser by applying a. test condition to the condenser and to a i standard `condenser of know-n and negligible dielectric absorption characteristic which may comprise short circuitingthecondensers or` applying a fixed potential thereto for a predetermined periodof time and thereafter momentarily applying a different potential or a short circuit, as the casemay be, to both of the condensers for a period of time in excess of the time consta-nts thereof and immediately thereafter connecting the condensers in parallel, the. parallel connection including a high sensitivity electrostatic voltmeter whereby the decrease in potential of the condenser under test withv respect to the potential of the standard condenser is continuously made manifest. A circuit arrangement suitable for achieving this result is also disclosed and described in detail.

Obviously many modifications and variations of the present invention are possible inthe light of the above teachings. lt is therefore to be understood that within the'scopeofv the appended claims the invention may be practiced otherwise. than as specifically described.

' What is claimed as new and desired to be secured by ltetters Patent of the UnitedStates is:

` li. 'A device for testing vthe dielectric absorption of a condenser comprising ay standard condenser for comparison, -a condenser to be tested, a source of voltage,

voltage` .dividing means connected tosaid sourceot voltage,..rneans including a,.rst switchsettable, to` different.

settings .and connected to, saidvoltage .dividing meansfor selecting.A av voltage, to.. he applied. to` said, condensers, in accordance with theV instantsetting of. the switchI means, a storage condenser, a. second switch meansconncctedto said. voltage. divider for selecting a second voltage to chargesaid storage. condenser, ,means for-sequentially connecting said. standard andtesting, condensers in. parallel for. simultaneouscharging from. the voltage selected by` said irst switch, means. for thereafter connecting said standard and-test condensers momentarily to said storage condenser, means for disconnecting said standard and testing condensers fromsaid storage condenserand.rneans. for thereafterconnecting., said. standard and said tested. condensers in parallel,- said` last-named meansincluding an. electrostatic voltage measuringzdevicewhereby the. difference in the voltages of said standardand. tested-icondensersduejtodielectc absorption-of the tested condenser isl continuouslyv made manifest.

2. Apparatus for testing the dielectric absorption of a high .precision condenser which' comprises a standard condenser of low dielectric absorption characteristic and of thesamecapacity asf'thetested, condenser, a storage condenser of Agreater capacity than the capacity of said standard and tested condensers, means for charging said storage condenser to a. predetermined potential, means for applyinga diierent potential simultaneously toy saidstandard and tested condensers, means for disconnecting said standard` andftested condensers. from said different potential and immediately thereafter momentarily connectingsaidstandard and-tested-condensers to said storage condenser; ahigh sensitivity electrostatic voltmeter, and

- means for connecting said standard and tested condensers in. series` opposition-v tosaid.voltmeter'v when the standard and. testedf'condensersi havebcen disconnected from said storage condensery whereby the. dielectric absorption of said testedcondenser may be made manifest on-said voltmeter 'as a differential value of voltage betweeny said standardr and tested condenser-s.

3. In a'circuit of the character. disclosed for testing and measuring the dielectric absorption of a test condenser comprising a voltage divider adapted for connectionto a source of D. C. voltage, means including a multi-contact switch connected to said voltage divider for selecting-a predetermined value of said voltage, a storage condenser connectedV to the movable element of said switch and. to one terminal ofsaid. Voltage divider for storing-al charge` thereon corresponding to` said predetermined voltage,.a second switch having a plurality of contactsv connected to `saidvoltagedivider for selecting a second value of said voltage, a third-switch having a plurality of wipers movable from an initial home position to aY final rest position, means for releasably lockingl said third switch in said initial position, resilient means for quickly operating said third switch to said final position as the `switch is unlocked, a standard condenser connected to one wiper of said third switch, means for connecting said test condenser to a second wiper of said third switch, means-including a third wiper of said third switch for connecting together the corresponding terminals of said test and standard condcnsers while the switch is in said` initial position, means for applying said different voltage from the moving element of said second.

said first and second wipers of the thind switch for establishing a series circuit between said standard and test condensers and said voltmeter where-by the loss of voltage of the test condenser by dielectric absorption is made manifest as a voltage differential between said standard and test condensers.

4. A claim according to claim 3 in which the momentary connection of the standard and test condensers to the storage condenser is established for a period of time in excess of the time constants of the charging circuits for said test and standard condensers.

5, A claim according to claim 3 in which said storage condenser is charged to a value of 600 to 625 volts.

6. A claim according to `claim 3 in which the capacity of said standard and test condensers each is 1 microfarad.

7. A claim according to claim 3 in which the dielectric material of the standard condenser is composed 'of polystyrene.

8. A claim according to claim 3 in which the charge and discharge circuits to the test condenser includes a resistance element thereby to prevent electrical oscillations as the condenser is charged and discharged respectively.

9. A claim according to claim 3 in which the rst named switch is provided with contact means connected to said storage condenser for maintaining the storage condenser normally completely discharged.

10. A claim according to claim 3 in which the second switch comprises contaict means connected to said standard and test condensers tfor normally maintaining the standard and test condensers completely discharged.

11. A method of -testing a condenser for dielectric absorption comprising the steps of simultaneously applying a lirst voltage to a standard polystyrene condenser and to the condenser being tested, isolating both condensers from said irst voltage, momentarily applying a second voltage simultaneously to both orf said condensers for a time in excess of the time Iconstants of the condenser charging circuits, said second voltage being of different value from said rst voltage, isolating said condensers from said second voltage and immediately thereafter connecting an electrostatic voltmeter between said condensers in such manner that the loss of voltage of the test condenser due to dielectric absorption therein is made continuously manifest as a voltage diterential between said condensers.

References Cited in the tile of this patent UNITED STATES PATENTS 2,177,569 Jorgensen et al Oct. 24, 1939 2,408,727 Blitz Oct. 8, 1946 2,455,543 Williams Dec. 7, 1948 

